Heretofore, a major drawback of compression molded thermoset glass fiber reinforced plastics (FRP) was that they had surface imperfections such as pits, pores, surface cracks, waviness and sink marks.
The in-mold coating process of U.S. Pat. No. 4,081,578 generally overcomes these imperfections by molding a low viscosity thermoset on top of the FRP in a second molding operation. The composition described in U.S. Pat. No. 4,081,578 contains free hydroxyl as well as isocyanate groups that co-react at room temperature, resulting in a limited (about one-half hour) pot life. In practice, the reactive ingredients are kept apart, and combined only immediately prior to application. This necessitates dual pumping equipment and an accurate metering device, which increase the cost and complexity of the system. A single component coating would thus offer a significant advantage. Moreover, conductive carbon black does not dispense well in isocyanate based in-mold coating compositions such as set forth in U.S. Pat. No. 4,081,578, and it is difficult to obtain after in-mold coating and subsequent electrostatic painting, a paint layer which is even.
While U.S. Pat. Nos. 4,414,173, 4,515,710 and 4,534,888 to Cobbledick substantially overcame the above-noted problems, good coverage was obtained only at high coating weights.